The present invention relates to a shadow mask type color cathode ray tube, and particularly to a color cathode ray tube having a shadow mask having improved workability in welding a skirt portion of a press-formed shadow mask to a support frame and improved magnetic resistance characteristics of the welded shadow mask.
Generally, in a shadow mask type color cathode ray tube, a shadow mask is positioned adjacent to a phosphor screen formed on an inner surface of a faceplate of its panel portion with a support frame having a skirt portion of the press-formed shadow mask fitted therein and spot-welded thereto, and suspended within the panel portion.
FIGS. 5A to 5C are respectively structural views showing one example of a shadow mask used for a known color cathode ray tube, FIG. 5A being a front view of the shadow mask, FIG. 5B being an enlarged side view in the vicinity of a welding point of a skirt portion, FIG. 5C being an enlarged sectional view of a portion from an imperforate portion to the skirt portion.
In FIGS. 5A to 5C, reference numeral 41 designates a shadow mask, 42 an apertured portion, 43 an imperforate portion, 44 a skirt portion, and X a welding point.
The shadow mask 41 has a curved apertured portion 42 having a multiplicity of electron-transmissive apertures, a curved imperforate portion 43 surrounding and integral with the apertured portion 42 and a skirt portion 44 bent back from a periphery of the curved imperforate portion 43 to form a rearwardly extending wall, and is integrally formed by press-forming a multi-apertured sheet-like metal blank.
Since the sheet-like metal blank constituting the shadow mask 41 is very thin, the forming properties during press forming are not always good, and in addition, since the strength of the metal blank is also relatively weak, the shape of the shadow mask 41 formed by press-forming is naturally limited. A corner portion of the shadow mask 41 is bent back smoothly from the curved imperforate portion 43 to blend into a rearwardly extending skirt portion 44, or is bent back from the curved imperforate portion 43 with one or more linear steps to blend into the rearwardly extending skirt portion 44, and side portions of the shadow mask are bent back from the curved imperforate portion 43 with a relatively small radius to blend into the rearwardly extending skirt portion 44. The side portions of the skirt portion 44 of the shadow mask 41 flare and curl outwardly by a distance .DELTA.S with respect to the boundary between the imperforate portion 43 and the skirt portion 44 as shown in FIG. 5C. The press-formed shadow mask 41 is fixed to a support frame (not shown) by fitting the skirt portion 44 of the shadow mask 41 within the support frame (or around the support frame in rare cases) and then spot-welding the skirt portion 44 to the support frame at several positions. The welding points between the skirt portion 44 and the support frame include, for example, one at two respective long sides of the shadow mask 41, one at two respective short sides, and one at the corners, as indicated by X in FIG. 5B.
In the known shadow mask 41, the curl .DELTA.S of the skirt portion 44 tends to become large when it is press-formed. There is a problem in that when the curl .DELTA.S exceeds an allowable value, the curl .DELTA.S interferes with the fitting of the shadow mask 41 into the support frame, resulting in a poor workability during the fitting operation.
There is a further problem in that in the known shadow mask 41, if the skirt portion 44 having a large .DELTA.S is forcibly fitted in the support frame, a stress applied to the skirt portion 44 is transmitted to even the imperforate portion 43 and the apertured portion 42 so that the curved shape of the apertured portion 42 of the shadow mask 41 is deformed, and the color selection property of the shadow mask 41 is degraded.
Still a further problem is that in the known shadow mask 41, if the skirt portion 44 has a large curl .DELTA.S, when the skirt portion 44 is fitted in the support frame, the intimate contact between the skirt portion 44 and the support frame cannot be obtained, and the magnetic resistance between the skirt portion and the support frame increases so that the magnetic resistance of the shadow mask 41 is deteriorated, and the color selection property is degraded under the influence of the earth's magnetic field.